THE LUNGS (See also pp. 160-165.)
The lungs of the dog obtain their distinctive appearance from the deep fissures that divide the lobes, sometimes so completely that they remain connected by little more than the branches of the bronchial tree and pulmonary vessels.
In consequence, torsion of a lobe is a possible complication of thoracic trauma, perhaps most frequently seen after traffic accidents. In contrast, lobulation is not evident to the naked eye through the covering pleura. The right lung, always somewhat the larger, possesses cranial, middle, caudal, and accessory lobes (Figure 13-11, A-B); the left one has only a divided cranial lobe and a caudal lobe. In keeping with the difference in size, the cardiac impression on the medial surface of the left lung is shallower than that on the right. Despite the existence of a small notch between the two parts of the cranial lobe, the left lung, for all practical purposes, may be regarded as covering the lateral face of the pericardium. The notch on the right side, between the cranial and middle lobes, is larger, although it is restricted to the ventral part of the fourth intercostal space; it provides the site recommended for heart (right ventricular) puncture and for ultrasonic cardiac imaging.Pulmonary ligaments connect the hilar region of the left lung to the aorta and that of the right lung to the esophagus, which it follows to the hiatus in the diaphragm.
The fields for auscultation and percussion of the lungs are triangular: the cranial border is provided by the fifth rib (actually the caudal border of the triceps), the dorsal border is provided by the lateral margin of the back muscles from the fifth rib to the eleventh space, and the basal border is provided by the line joining the sixth costochondral junction, the middle of the eighth rib, and the dorsal end of the eleventh space. The forelimb may be drawn forward to increase the accessible area by the space of a couple of ribs.
In plain radiographs, the principal features of the lungs are made by the vessels and bronchi. The blood within the arteries and veins, which cannot be immediately differentiated, produces a pattern of light streaks radiating from the hilar region toward the periphery, branching and tapering as they go. The bronchi, being filled with air, provide dark streaks that contrast less definitely with the lung parenchyma. Their walls may be invisible or appear as narrow, whitish lines, especially in older animals in which the cartilage tends to have calcified. The relationships within the bronchial-vascular triads vary in different regions and in different radiographic views. The components are most clearly depicted when portrayed end-on; the dark circles of the bronchial lumina are then flanked by white circles representing the companion vessels. The subpleural connective tissue bordering the interlobar fissures may appear as fine lines when penetrated tangentially.
Both the bronchial tree and the pulmonary vasculature may be made more evident by the use of an appropriate contrast technique (contrast bronchography: Figure 13-10; angiocardiography: Figure 13-21). The larger divisions of the bronchial tree are then very clearly depicted, and if the normal pattern of branching is known, any deviation may reveal the existence of pathology. A more exact picture of the nature and extent of that pathology may obtained by the use of bronchoscopy, which also requires familiarity with the branching pattern. The principal bronchi produced at the bifurcation of the trachea are separated by a sharp ridge, the carina. The bronchi that initially branch from the principal bronchi supply the different lobes and are named accordingly. The divisions of the next order, the segmental bronchi, also arise according to a consistent pattern and are each associated with well-defined parts of the lobes. Subsequent divisions into smaller bronchi are less regular and less predictable.
The parts of lung associated with the segmental bronchi (the bronchopulmonary segments) constitute the divisions of the lungs on which surgery is based. Various systems of nomenclature have been devised for the identification; one is based on topography, while a second employs a basically numerical code; their details may safely be left to those with a specialist interest.The manner of branching is such that after each division the two daughter bronchi in combination provide a greater cross-sectional area, thus offering less resistance to the flow of air than their parent; resistance thus decreases progressively as air passes more deeply into the lung. This process is a continuation of that in the upper respiratory tract, where the nostrils, the nasal cavity, the pharynx, the larynx, and the trachea offer successively less obstruction than the preceding segment. According to one estimate, the resistance to the inspiratory airflow in dogs is 79% due to the nasal, 6% to the laryngeal, and 15% to the bronchopulmonary parts of the tract; the corresponding figures at expiration are given as 74%, 3%, and 23%, respectively. These findings offer a reminder that dogs of brachycephalic breeds in which nasopharyngeal resistance is pronounced may be severely compromised even when breathing normally.
The lungs of the cat, relatively shallow in comparison with those of the dog, show no significant differences in general or radiographic anatomy or in bronchopulmonary segmentation.